In semiconductor manufacturing processes, numerous layers may be deposited onto a semiconductor substrate to form, for example, electronic devices and/or circuits. In some cases, such layers are protected against environmental influences, such as, for example, moisture. The layer to be protected may be termed an underlying layer and may be, for example, the substrate itself, a grown layer (such as an oxide), or a deposited layer, among other possibilities. To protect such underlying layers, a protecting layer, which may also be referred to as a covering layer, is deposited on top of an underlying layer so as to protect (e.g. cover) the underlying layer.
Many advanced semiconductor processes also have a need for insulating materials with low dielectric constants (also called k-values). This need arises from the fact that the operational speed of circuits manufactured with such processes is dependent on the impedance of signal lines in the circuit (such as copper lines) and, thus, the dielectric constant of the signal lines and any insulating material surrounding the signal lines. High performance circuits, therefore, have a need for ever-improved materials. Presently, it is preferable that such materials have dielectric constants of 2.0 or lower. Development of such low-k dielectric materials is motivated by a desire to reduce capacitance, e.g. capacitance between interconnects and signal lines in ultra large-scale integrated circuits (ICs), as such capacitance is typically adverse to circuit performance. Furthermore, reducing such capacitance also reduces crosstalk (e.g. signal interference) between adjacent signal lines, an increasingly severe problem in advanced processes due to the close proximity of such signal lines to one another. Thus, low-k dielectrics may be used in a broad spectrum of applications that are sensitive to ever-smaller geometries.
One concern with low-k dielectrics is the permittivity of such materials. In this regard, low-k dielectric materials (e.g. organic or inorganic) with relatively low permittivities can exhibit a constitutive porosity and/or a subtractive porosity. Constitutive porosity is typical for materials with k-values on the order of 2.7. Subtractive porosity is typical for materials with a k-value smaller than 2.7, though exceptions may exist. In this respect, materials with low relative permittivity can be very sensitive to the environment to which they are exposed, be it the ambient (e.g. room) environment, the semiconductor processing environment or any environment to which such materials are subjected, whether deliberately or unintentionally. Such exposure could lead to problems with such materials. For example, unwanted process interactions, absorption of molecules, contamination during processing, and mobile ion drift diffusion may occur, as some examples. Such situations typically lead to degradation of such porous material layers, and can lead to an undesirable increase in the dielectric constant of those layers.
It is noted that such low-k dielectric materials are not necessarily porous. However, for non-porous materials, environmental factors, such as exposure to any number of materials (e.g., solvents) during semiconductor processing may cause certain materials to swell, which may damage and/or deteriorate their electrical, physical, chemical and/or mechanical properties. Such damage and/or deterioration may occur, even though the compound to which the layer is exposed does not penetrate into the material.
To avoid a change in a material's k-value due to absorption or adsorption of moisture, gases, liquids, or any other substances during, or between processing steps, the low-k material may be encapsulated (sealed) by one or more covering layers. It is desirable that such a covering layer is applied to substantially the entire exposed surface of such a low-k dielectric material layer.
A covering layer can be deposited as a thin film on the exposed surface of a low-k layer (or other underlying layer) or by treating a porous material to effect pore sealing using, for example, plasma treatment, chemical treatment, deposition of a self assembled monolayer, or from an etch process. In this regard, etching of vias and/or trenches can result in pore sealing. This is due, at least in part, to the plasma treatment and the chemical nature of the plasma. As such, etching as used in the context of the present invention includes plasma treatment and chemical treatment. It is desirable to keep the covering layer as thin as possible in order to keep the overall dielectric constant (of the underlying layer in combination with the covering layer) as low as possible. If the covering layer is too thick, an increase of the dielectric constant will occur.
Porous materials are often used because of their low k-values. It is desirable to keep covering layers on such materials as thin as possible so as not to substantially increase the overall k-value. However, one undesirable consequence of keeping the covering layer on such materials as thin as possible is that the covering layer will be discontinuous and, as a result, certain parts of the underlying layer will not be covered with the covering layer. In this situation, such discontinuities of the covering layer (or the presence of defects in such a layer) may lead to the exposure of the underlying layer (e.g., a low k dielectric layer,) to environmental factors, as was discussed above. Therefore, degradation of the electrical, physical, chemical and/or mechanical properties of the underlying layer may occur in this situation. While thicker covering layers have lower defect rates, they result in an undesirable increase in the overall dielectric constant. Such an increase in the overall dielectric constant may adversely affect the performance of a circuit including such dielectric/covering layer combinations. In view of the foregoing, it is desirable that covering layers are sufficiently thin, yet substantially free from discontinuities and/or defects. In this regard, techniques that are able to monitor for such defects in the covering layer during processing are desirable.
It is also desirable that these techniques be non-destructive, so that such a monitoring technique may be used for quality control purposes in semiconductor manufacturing process. In this regard, if discontinuities and/or defects are detected in a covering layer, another covering layer may be applied on top of the discontinuous/defective covering layer. Thereafter, such a non-destructive monitoring technique could be employed once again to examine the second covering layer for defects.
Alternatively, such techniques may be used to reject defective materials being processed in a semiconductor processing line. In this case, such defect monitoring techniques could be used to identify and remove, from a semiconductor processing line, electronic devices and/or semiconductor wafers in which covering layers show discontinuities and/or defects, whereas devices and/or wafers without defects in the covering layer are processed further.